High-power laser systems are utilized for a host of different applications, such as welding, cutting, drilling, and materials processing. Such laser systems typically include a laser emitter, the laser light from which is coupled into an optical fiber (or simply a “fiber”), and an optical system that focuses the laser light from the fiber onto the workpiece to be processed. The optical system is typically engineered to produce the highest-quality laser beam, or, equivalently, the beam with the lowest beam parameter product (BPP). The BPP is the product of the laser beam's divergence angle (half-angle) and the radius of the beam at its narrowest point (i.e., the beam waist, the minimum spot size). The BPP quantifies the quality of the laser beam and how well it can be focused to a small spot, and is typically expressed in units of millimeter-milliradians (mm-mrad). A Gaussian beam has the lowest possible BPP, given by the wavelength of the laser light divided by pi. The ratio of the BPP of an actual beam to that of an ideal Gaussian beam at the same wavelength is denoted M2, or the “beam quality factor,” which is a wavelength-independent measure of beam quality, with the “best” quality corresponding to the “lowest” beam quality factor of 1.
Wavelength beam combining (WBC) is a technique for scaling the output power and brightness from laser diode bars, stacks of diode bars, or other lasers arranged in one- or two-dimensional array. WBC methods have been developed to combine beams along one or both dimensions of an array of emitters. Typical WBC systems include a plurality of emitters, such as one or more diode bars, that are combined using a dispersive element to form a multi-wavelength beam. Each emitter in the WBC system individually resonates, and is stabilized through wavelength-specific feedback from a common partially reflecting output coupler that is filtered by the dispersive element along a beam-combining dimension. Exemplary WBC systems are detailed in U.S. Pat. No. 6,192,062, filed on Feb. 4, 2000, U.S. Pat. No. 6,208,679, filed on Sep. 8, 1998, U.S. Pat. No. 8,670,180, filed on Aug. 25, 2011, and U.S. Pat. No. 8,559,107, filed on Mar. 7, 2011, the entire disclosure of each of which is incorporated by reference herein.
A variety of WBC techniques have been utilized to form high-power lasers for a host of different applications, and such techniques often involve the formation and manipulation of beams having fast and slow diverging axes. One of the problems that may arise when using an optical rotation system (or “optical rotator,” or “beam twister,” or “optical twister”) to individually rotate each beam emitted by an array of emitters is that the final combined beam may have a skewed or twisted profile. This may be caused in part when one of the axes of the beam (i.e., fast or slow) continues to diverge through the optical rotation system, which may include or consist essentially of two (or two arrays of) cylindrical lenses. Placement of a fast axis collimating (FAC) lens before the optical rotation system typically sufficiently collimates the fast diverging axis (FA) such that the beam may fully rotate through optical rotation system. The FAC lens is generally able to sufficiently collimate the FA due to the rapid divergence of the beam(s) along the FA. However, the slow diverging axis (SA) generally does not diverge as quickly, so lenses downstream of the optical rotation system have been used to collimate the beam and help minimize the M2 value, which is desired for WBC systems. Furthermore, the slow divergence and size of the emitted beams along the SA are generally addressed via a “far field” optical solution downstream of the optical rotation system. Thus, there is a need for optical systems and methods for WBC systems that reduce divergence along the SA via components disposed upstream of an optical rotation system.